Hard disk drives are used in most modern computer systems to store and retrieve programs and data. The hard disks are magnetic disks which are permanently enclosed in the hard disk drive to prevent contamination. Generally, the hard disk drive includes a spindle on which the disks are mounted and rotated with a selected angular velocity. The hard disk drives include a magnetic head that is translated across the surface of the disk to allow for access to a selected annular track. The magnetic disks are typically journaled for rotation about the spindle of the hard drive in a spaced relationship to one another. A tracking arm is associated with each disk and the read/write head is mounted to this tracking arm for accessing the desired information. These magnetic heads are typically referred to as "flying" data heads because they do not contact the surface of the disk during rotation. Rather, the magnetic heads hover above the surface on an air bearing that is located between the disk and head which is caused by rotation of the disk at high speeds.
A persisting problem with rigid magnetic memory disks is that asperities, which are essentially protrusions on the surfaces of the disks, may cause an anomaly when encountered by the head during high speed revolutions. These asperities can cause errors in the transfer of information or even damage to the head. In effort to reduce the occurrences of asperities, manufacturers commonly burnish the memory surfaces of the disk to remove asperities. In the burnishing process, a burnishing head, rather than a magnetic read-write head, is mounted in a similar manner relative to the disk as discussed above. Burnishing heads may be designed as either "flying" heads which pass over the surface to be burnished or they may be designed as "contact" burnishing heads which have a contact surface that directly engages the asperities. During the burnishing process, the burnishing head operates to smooth out the surface protrusions.
The next step in further refining magnetic (or optical) disks for production is detecting any unwanted asperities which remain after the burnishing operation and is accomplished through the use of a glide head. The purpose of a glide head is to detect, via proximity or by contact, any remaining asperities which may come into contact with the write data head during use. Glide heads are, thus, required to hover and detect asperities which are located above specified data head flying heights. Glide heads dynamically test the integrity of a disk's surfaces.
The continuous trend in the magnetic media industry is towards requiring magnetic recording disks to have ever increasing recording densities. Accordingly, for manufacturers to develop production quality rigid memory disks for use in this industry and the computer industry in general, it is necessary to utilize glide heads that have more sensitive response characteristics. Existing glide heads have inherent problems associated with them because it is difficult to precisely control the electrical response characteristics of these devices.
The electrical response of a glide head is dependent upon detection parameters of amplitude, frequency, and signal to noise ratio (S/N). However, because the industry's demands for higher magnetic densities requires a lowering of the data head's flying height over the surface of the magnetic disks, it becomes more difficult to tighten the physical tolerances of glide heads and effectively control the frequency, amplitude and signal to noise ratio. Current glide head designs, for example, rely predominantly on the function of an accelerometer to control these detection parameters. Unfortunately, these designs are becoming less effective at detecting asperities as demands increase and they are increasingly susceptible to physical and thermal stresses during shipping and use.
In the past, it has been known to employ a glide head, whose slide component is that portion of the glide head which directly contacts the surface asperities, that is configured to include a lateral wing portion that has a layer of piezoelectric material adhered thereto. As the slider comes into contact with a surface asperity, it leads to an excitation of all the natural internal vibrations of the glide head/PZT assembly. The particular disturbances of the PZT sensor causes a voltage output from the crystalline lattice of the piezoelectric material. Part of this electrical signal, in the frequency window of the electronic filter in use, is then monitored as an r.m.s (root-mean-square) value. Typically, one sets a threshold r.m.s voltage over which discs are rejected for improper surface finish. The problem is that since all glide heads are manufactured with a certain geometric tolerance, they all have a different transfer function, i.e., they all respond differently in both frequency and amplitude to a given impact asperity. It is therefore critical to calibrate precisely the response of each individual glide head.
The current glide technology uses the glide head and piezoelectric sensor to detect a signal upon head-disk contact. The detection system is traditionally calibrated by utilizing a specially made "bump disk" which has asperities of desired height and size that protrude out of a flat disk surface. The asperities are either deposited via sputtering techniques or formed by laser texturing techniques, for example. A glide head is then flown over the bump disk. By gradual reduction of the disk spinning velocity, the glide head is brought closer to the disk and eventually comes into contact with the asperity. The onset of contact, as detected by the piezoelectric sensor, defines the specific disk spinning velocity for the head to fly at the desired height.
One of the problems with this calibration technique, however, is that the calibration may be affected by a number of different factors. These include the asperity integrity, the glide head flying characteristics, the quality of the piezoelectric sensor, and the transfer function. The combined effects of these different factors are complex and extremely difficult to decouple.